Rings are used to seal between a reciprocating piston and a cylinder. Conventionally, the rings are placed in grooves in the piston. In FIG. 1, a piston 100 is shown viewed from the top. Piston 100 has a piston skirt extending downwardly from the piston top. The piston skirt is generally cylindrical and has grooves formed in the outer surface. The inward edge of the groove is shown as a dashed circle 102. A piston ring 104 is shown that is spread apart to form a gap 106.
The situation when piston ring 104 is allowed to contract into the groove in piston 100 and into the cylinder (not shown) is shown in FIG. 2. Gap 106 is much smaller in the configuration in FIG. 2 than in the expanded position shown in FIG. 1. It is advantageous that gap 106 is very small when ring 104 is installed into the cylinder (not shown) so that a minimal amount of gases can pass through gap 106.
In a two-stroke engine, it is undesirable to lose oil through the ports both from an oil consumption and emissions perspective. Thus, two-stroke engines are provided with an oil ring that is located either in the lower portion of the piston i.e., below the ports or in the cylinder wall below the ports at TDC. A disadvantage of having the oil ring in the piston is that the engine is longer in the direction along the cylinder liner axis. In an OPOC engine, such as disclosed in U.S. Pat. No. 6,170,443, which is incorporated herein in its entirety, the extra length is in the longest dimension of the engine. A stationary seal can be a challenge to install the seal with a small gap in the cylinder liner.
An example of an OPOC engine 10 is shown in FIG. 27. An inner piston 12 and an outer piston 14 are associated with a right cylinder (not shown to view the pistons) and an inner piston 13 and an outer piston 15 are associated with a left cylinder (not shown). Pistons 12 and 13 couple to crankshaft 20 via pushrods 16 and pistons 14 and 15 coupled to crankshaft 20 via pullrods 18 (a pair of pullrods 18 for each outer piston). Inner pistons 12 and 13 have a groove near the bottom of the piston. In contrast, outer pistons 14 and 15 have only grooves for compression rings and no groove to accommodate an oil sealing ring. The oil sealing rings are disposed in grooves in the cylinder liners, not shown in FIG. 27.
A sealing ring, which may serve as a stationary oil ring mounted into the cylinder liner, is shown in FIGS. 3 and 4. A cylinder 120 has a groove formed in an inner surface of cylinder 120. The base of the groove is shown as a dashed circle 122 (hidden line in the view shown in FIG. 3). A ring 124 with a small gap 126 is shown inside cylinder 120. When ring 124 is expanded to fit inside the groove in cylinder 120, gap 126 becomes intolerably large. That is, the gas flow through gap 126 would be too great.
In one alternative of a stationary seal, the cylinder is formed in two parts which couple axially. The groove is located at the end of one cylinder part. The ring is installed into the groove before the two parts of the cylinder are assembled. Disadvantages of this configuration include: higher part count: 2 pieces plus bolts for assembly; extra material to accommodate the bolts; and the requirement that the two pieces be perfectly aligned during machining and final assembly to provide a completely collinear cylinder liner in which the piston may freely reciprocate.
It would be desirable to have a ring that could be installed in groove in a single-piece cylinder, but without a large gap.